A mobile communication system is formed principally from, for example, mobile vehicle-carried communication apparatus, mobile portable communication apparatus or like apparatus (hereinafter referred to as mobile stations) and radio base stations which communicate with the mobile stations by radio channels. In such a mobile communication system, sharing of a same radio frequency spectrum by different radio systems (such sharing is hereinafter referred to as frequency channel sharing) is sometimes performed between a frequency division multiple access (hereinafter referred to as FDMA) or a TDMA system and a CDMA system. In the CDMA system, frequency channel sharing between different codes have already been put into practical use.
Here, a mobile communication system of the time slot sharing type where a TDMA signal and a time-divided CDMA signal are shared in the same time slot is disclosed in U.S. patent application No. 08/524,974, now U.S. Pat. No. 5,805,581 incorporated by reference herein. However, in this patent application, measurement of the distance between a mobile station and a radio base station is not mentioned, and no description of synchronization between radio base stations is given either.
Meanwhile, also a technique regarding a mobile communication system of the time slot sharing type and simultaneously of the frequency channel sharing type and another technique regarding synchronization between radio base stations which employs a semi-fixed communication apparatus (hereinafter referred to as WLL station) which employs a wireless local loop (hereinafter referred to as WLL) or a like apparatus are present already. However, those techniques don't involve measurement of the distance between a mobile station and a radio base station or synchronization between radio base stations by a chip rate unit.
A mobile communication system in which a plurality of CDMA signals are shared is known by U.S. Pat. No. 5,363,403. However, the specification of U.S. Pat. No. 5,363,403 describes nothing of a method of handling a time-divided CDMA signal. Another mobile communication system is known by U.S. Pat. No. 5,511,068. However, U.S. Pat. No. 5,511,068 relates to an adaptive filter for a time-divided CDMA signal system, and the specification of U.S. Pat. No. 5,511,068 does not describe that frequency channel sharing of a CDMA signal and a TDMA signal is performed in one time slot.
Meanwhile, a mobile communication system disclosed in International Publication WO96/18277 relates to a macro diversity technique, and also an exchange is a simple one which merely compares service qualities of information received simultaneously from a plurality of radio base stations to select a best one of them, but the results are not utilized for hand-over or the like. Further, the specification of International Publication WO96/18277 describes nothing of a combination with a time-divided CDMA technique.
Further, a mobile communication system which measures the distance between a mobile station and a radio base station using a chip code is disclosed by U.S. Pat. No. 5,506,864 and U.S. Pat. No. 5,365,544. However, the U.S. patents relate to a mobile communication system of the continuous CDMA system, and while the specifications of them describe that the mobile communication system is characterized in that it uses a generic chip code in addition to a spread-spectrum chip code, they do not relate to a mobile communication system which is based on the time-divided CDMA system and do not mention that the distance between a mobile station and a radio base station is measured making use of a frame structure or a characteristic of the time-divided CDMA system.
Meanwhile, frame synchronization between radio base stations in a mobile communication system is known by U.S. Pat. No. 5,440,561. However, the specification of the U.S. patent merely describes a TDMA system but does not describe a time-divided CDMA system. Also in regard to arrangement of radio base stations, it is presumed that a radio base station is located at the center of a hexagonal cell, but no investigation is made for such a case that the distances between radio base stations are not equal to each other.
It is to be noted that, as other documents which relate to frame synchronization between radio base stations of a mobile communication system, Japanese Patent Laid-Open Application No. 8-23569 and Japanese Patent Laid-Open Application No. 8-33037 as well as Japanese Patent Laid-Open Application No. Hei 7-46660 are known.
Each of Japanese Patent Laid-Open Application No. 8-23569 and Japanese Patent Laid-Open Application No. 8-33037 mentioned above describes, in the description of [claims] in the specification thereof, that "so that the difference in timing may be equal to n times the one burst length of TDMA/TDD". This is a characteristic of the TDMA system which employs satellite communication, and has no relation to a mobile communication on the ground. In particular, in a mobile communication system on the ground, since the distance between a mobile station and a radio base station does not become so long as the distance between a communications satellite and the earth, such a situation as described above does not occur, and also such a condition that a radio base station transmits in a certain frame from among multiple frames whereas a tributary radio base station transmits in another frame does not occur. Furthermore, the specifications mentioned above describe nothing of frame synchronization by a chip plate unit of the time-divided CDMA system.
Meanwhile, Japanese Patent Laid-Open Application No. Hei 7-46660 discloses that information representing to which one of time slots in a TDMA frame a TDMA control signal is transmitted is included in control information. Indeed where the frame length is long or in a system which requires provision of several control information time slots, such a prescription is required. However, where a mobile communication system which presupposes that a control signal is transmitted and received using a first time slot in a frame is considered, information indicating this explicitly need not be included in a control signal. Furthermore, a mobile communication system which can set a plurality of TDMA control channels and a plurality of time-divided CDMA control channels in a first time slot need not use the technique disclosed in Japanese Patent Laid-Open Application No. Hei 7-46660 mentioned above.
In this manner, as a mobile communication system which includes a plurality of mobile stations and one or more radio base stations which communicate with the radio stations by radio channels, such various systems as described above are conventionally known already. In such radio communication systems, where both of the TDMA system and the time-divided CDMA system are used, it becomes necessary to establish frame synchronization on the time axis between different radio base stations.
It is to be noted that, while such establishment of frame synchronization between radio base stations can be performed using a time indication signal of the global positioning system (hereinafter referred to as GPSP), to this end, it is required that all radio base stations be equipped with a GPS reception system and besides those radio base stations are installed at locations at which a GPS signal can be received.
However, in a personal communication system (hereinafter referred to as PCS) and so forth, a radio base station is sometimes installed indoors or in underground facilities. In order to utilize a GPS signal also in such a case, a complicated apparatus is required. The conventional mobile communication systems have such problems as described above.
The present invention has been made to solve such problems as described above, and it is an object of the present invention to provide a mobile communication system that a control channel is allocated to a TDMA signal or a time-divided CDMA signal to establish frame synchronization between a plurality of radio base stations such as a plurality of operators (such a communication user which owns a system which supplies a mobile communication service is normally called an operator) and the distance between a mobile station and a radio base station is measured accurately so that discrimination of hand-over can be performed.